The overcurrent protection power supply apparatus shown in FIG. 5 is disclosed in Japanese Patent No. 3706515.
The overcurrent protection power supply apparatus 101 shown in FIG. 5 comprises a switch circuit 102, a control circuit 105, an overcurrent detecting circuit 108, multi-source FET 109 and a temperature sensor 112. The multi-source FET 109 has N type metal oxide semiconductor field effect transistors (MOSFETs) 111 and 110. The temperature sensor 112 detects the temperature of the MOSFET 111.
Such a overcurrent protection power supply apparatus 101 is used as a power supply apparatus that supplies electric power to a load from a relatively low voltage DC power source 117 (for example, a 12V or 24V DC power source installed in a vehicle).
This known overcurrent protection power supply apparatus 101 operates as follows.
When the switch 103 is turned on, the control circuit 105 turns on the MOSFETs 110 and 111, so that electric power is supplied from the DC power source 117 to the load 116.
If the current ID flowing through the MOSFET 111 becomes an overcurrent, the voltage VDS between the drain and the source of the MOSFET 111 increases, and the current detection voltage VB decreases and becomes less than a reference voltage VA (VA>VB). As a result, the output of a comparator 107 becomes a high level. That is, an overcurrent detection signal is output from the comparator 107. When the overcurrent detection signal is output from the overcurrent detecting circuit 108, the control circuit 105 turns off the MOSFETs 111 and 110.
Furthermore, when it is determined by using a temperature signal from the temperature sensor 112 that the temperature of the MOSFET 111 exceeds a predetermined temperature, the control circuit 105 turns off the MOSFETs 111 and 110.
For example, in case the insulating film of the gate of the MOSFET 111 or the MOSFET 110 is partially damaged, a leakage current may flow between the gate and the source. When the leakage current increases, the voltage drop across resistor 106 increases. As a result, the source-gate voltages of the MOSFET 111 and the MOSFET 110 increase. Thus, the MOSFETs 111 and 110 generate heat due to the increase of the on-resistance. In this situation, the current ID does not increase, and it may even slightly decrease. Therefore, an overcurrent detection signal will not be output from the overcurrent detecting circuit 108. As a result, the heat generation state of the MOSFETs 111 and 110 continues and there is a risk of damage. The temperature sensor 112 is provided in order to protect the components against such an overheating.